I have sideloaded quite a few books using Calibre. These are DRM books in ePub format.

I download the .epub to my laptop. Open the .epub in Calibre and download metadata. All metadata is correct in Calibre, including book covers. Then I sideload them to my Nook Color.

Maybe 5% of them display the covers on the Nook.

I can't figure out what is different between the books that display the cover correctly on the Nook, and the ones that don't. Is there anything I can do to get my sideloaded books to display the correct cover graphic?

I'm not sure I understand what you mean. These are DRM books. If I click on the "Convert Books" menu option, I get a message saying "Connot convert ... This book is locked by DRM." So what steps can I take for DRM books? Thank you!

AFAIK, there is nothing you can do about cover display from a DRM'd book (if the book came from B&N you can bring that issue up with customer support). My library books seldom display cover art, but since I have them for <2 weeks, it's not a biggie for me.